1. Field
The present invention pertains to the field of satellite communications. More particularly, this invention relates to reducing service outages in a multibeam satellite system
2. Background
Satellites, or constellations of satellites, can provide communications virtually anywhere without costly infrastructure, such as cellular towers or landline connections to terminal users. These advantages make satellites ideal for a wide variety of applications, including high speed data communications. For instance, rather than limiting broadband Internet service to areas where Digital Subscriber Lines (DSL), digital cable, or fiber optics are available, a satellite-based system can provide comparable service to virtually every potential user, including urban, suburban, rural, or mobile users.
For example, a satellite in geosynchronous orbit can have a service area the size of the entire continental United States. The combined data traffic in the service area for broadband Internet access can consume a large volume of bandwidth, but bandwidth in satellite communications is limited by the available spectrum of radio frequencies. In order to have enough bandwidth, the satellite projects multiple “beams”, rather than a single beam, to cover the service area in a beam pattern similar to cell patterns used in cellular phone systems. Each beam reuses the available frequencies, or some portion thereof, to substantially increase the total bandwidth through the satellite.
The satellite “bounces” signals back and forth between terminals in a beam and a “gateway” that supports the beam. The gateway can direct communications among the terminals and between the terminals and the outside world. For instance, the gateway may include an Internet access point to connect the terminals in the beam to the Internet.
A beam can cover a large area and provide service to many terminals. A feeder link between a gateway and a satellite is essentially a signal stream narrowly focused in a line of sight between the gateway and the satellite. A feeder link supports all of the data traffic between the satellite for all the terminals in a given beam. If the signal quality for the feeder link degrades beyond a certain limit, the entire beam experiences a service outage. Thermal noise can degrade a feeder link.
One solution to the feeder link bottleneck is “spatial diversity.” Rain of sufficient intensity to cause a service outage is usually highly localized. The probability of a rain intense enough to cause a service outage at two diverse locations simultaneously is very low. Diversity is achieved using a redundant gateway at some diverse location, usually several miles away. Traditionally, diversity is done on a 1-for-1 basis. That is, each feeder link is supported by two gateways, one being held in “hot” standby in case the active gateway fails.
1-for-1 diversity is useful when dealing with a satellite system connected to a circuit-based network, such as a typical public switched telephone network (PSTN). In a satellite system, the intended recipient of a telephone call is associated with a particular beam. The call is routed through the PSTN network to a gateway serving that particular beam. When the gateway completes the call to the recipient terminal through the satellite, the “circuit” between the caller and the recipient is said to have been established. If the gateway then experiences a service outage, the circuit is broken and the call is lost. However, with 1-for-1 diversity, the “gateway” is actually two redundant gateways that are physically separated by a number of miles. One is active and the other is in “hot” standby mode. The two gateways appear as one network location in the telephone network so that if one gateway fails, the other gateway can immediately take its place without breaking the circuit.
One drawback to 1-for-1 diversity is cost. Gateways can be very expensive, including large antenna arrays, high speed routers, equipment for converting large volumes of data to and from radio frequency signals, and the like. A satellite system serving the continental United States may use many tens, or even hundreds, of gateway locations. 1-for-1 diversity for each gateway location can dramatically increase the cost of the system. It is especially important to keep costs, and hence subscriber rates, low in the highly competitive data services markets, such as broadband Internet access.